Sump Pump Generator Size Calculator (by HP)

Data reviewed 2026-07-05 ·how we calculate

The sump pump earns the cruelest spot in outage planning: the same storm that floods your basement is the one that cuts the power, so the pump dies at the exact moment it matters. The load itself is manageable — a 1/3 HP pump runs at about 800W with a 1,300W starting draw, inside reach of even small generators — and unlike well pumps, most sump pumps are 120V plug-in units, so no electrician is needed to power one through an outage. This calculator sizes it by horsepower and helps you weigh the generator against a battery-backup pump.

Size a generator for this load

Computed on typical values; ranges shown below. 25% headroom applied.

Generator safety — read every time

Never run a generator indoors, in a garage, or near windows, doors or vents.Carbon monoxide kills quickly and without warning — place the generator 20+ feet from the house with the exhaust pointed away, and use battery CO detectors inside.
Never backfeed your house through a wall outlet or dryer outlet. Backfeeding can electrocute utility line workers and destroy equipment when power returns.
Connecting a generator to home circuits requires a transfer switch or interlock installed by a licensed electrician. There is no safe DIY shortcut, and this site does not provide wiring instructions.
Use heavy, outdoor-rated extension cords (12-gauge or thicker), fully uncoiled, sized to the load and the distance.

How to size it step by step

Read the pump’s label (on the housing or the plug tag): horsepower and amps. 1/3 HP at roughly 800W running is the residential standard; use amps × 120V if the label gives current.
Size to the start: 1,300W typical for a 1/3 HP unit — and remember the pump restarts every cycle, so this surge repeats every minute or two during the storm that made you buy the generator.
Apply 25% headroom: 1,300W × 1.25 = 1,625W, within the 2,000W inverter class — one of the few genuinely critical loads that small.
Plan the whole storm picture: the fridge usually needs power at the same time. Both running draws plus only the fridge’s larger surge is the combined math — the home essentials + sump pump scenario walks through it.

Pro tips

Check your nameplate first — every figure on this page is a planning estimate, and the label on your specific unit beats any chart.
Test the pump before storm season: lift the float with a broom handle and confirm it runs — a seized pump makes every generator calculation moot.
Know your inflow: if the basement takes water within an hour of the pump stopping, pre-position the generator and cord BEFORE the storm, not during it.
A water alarm ($15, battery-powered) in the sump pit tells you the backup plan is needed while there is still time to act.

The data behind this calculator

Sump pump load figures by horsepower
Figure	Value	Source
1/3 HP running / starting	~800W / ~1,300W	Generac sizing chart; Zoeller M53 specifications
1/2 HP running / starting	~1,050W / ~2,150W	Generac sizing chart; Wayne/Zoeller specifications
Voltage	120V plug-in (GFCI-protected circuit typical)	US sump pump nameplates (NEMA 5-15 plug)
Storm cycling rate (heavy inflow)	one start every 1–3 minutes, for hours	Basement Health Association guidance; pump duty observations — planning figure

Duty cycle: Duty cycle is weather: in a heavy storm a sump pump can cycle every 1–3 minutes for hours, repeating its starting surge each time. Size and fuel-plan for storm duty, not fair-weather duty.

Sump pump generator questions, answered

What size generator will run a sump pump?

A typical 1/3 HP sump pump needs about 800W running and 1,300W starting. With 25% headroom that is 1,625W — a 2,000W inverter generator covers it, and the 3,500–4,500W class covers it alongside the refrigerator and lights, which is the realistic storm configuration. The 1/2 HP class (1,050W running, 2,150W starting) pushes a bare 2,000W unit to its surge limit; size up if that is your pump.

Is a battery backup sump pump better than a generator?

They solve different failure modes, and serious basements use both. A battery backup pump (a second pump on a deep-cycle battery) takes over instantly, unattended, at 3 a.m. — but typically pumps at reduced capacity for a limited number of hours. A generator runs the full-power main pump indefinitely but needs a human to deploy it. Battery for the first hours and small storms; generator for the long outage. If choosing one and your area gets multi-day outages, the generator protects more than just the sump.

How much fuel will the sump pump burn during a storm?

Cycling duty is kinder on fuel than it sounds: even at a punishing one 30-second run per minute, a 1/3 HP pump averages ~400W — about 10kWh over 24 hours, or very roughly 2–3 gallons of gasoline on a small inverter. The catch is you must be there to refuel; storms that flood basements often last longer than a tank. A dual-fuel unit on a 20 lb propane cylinder buys 8+ hours per bottle unattended by comparison.

Can I run the sump pump on a long extension cord to keep the generator away from the house?

Yes — the generator must be 20+ feet out and the pump is in the basement, so a long cord is the design, not a compromise. Make it 12-gauge minimum (10-gauge past 100 ft): every restart pulls the 1,300W surge through that cord, and voltage drop turns easy starts into stalls. Route it through a window with a board-and-notch spacer rather than pinching it in a door, and keep the connection ends out of any water path.

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